chore: remove extend dependency

[gameroman]

Checklist

• npm test passes (tip: npm run lint-fix can correct most style issues)
• tests are included
• documentation is changed or added

[CLAassistant]

All committers have signed the CLA.

[pyrooka]

Hey @gameroman, thanks for the contribution. We'll try to look into the PR soon, but I'd like to ask two things in the meantime:

1. Sign off all your commits in the branch, otherwise the DCO check will keep failing
2. Since the purpose of this PR is to remove the extend dependency from the project, it should be also removed from the package.json - along with package-lock.json -, right?